Synthesis, Characterization, and Plasmonic Enhancement of Fluorescent Carbon Nanodots

Abstract

Carbon nanodots are fluorescent nanoparticles that have unique photophysical properties that make them ideal candidates as luminescent probes for various fields of study. Although the synthetic routes of carbon nanodots have been extensively investigated, many of them produce carbon nanodots with low fluorescence quantum yields and/or require surface modifications to obtain fluorescent nanoparticles. This research discusses the development of a combustion-based method to synthesize carbon nanodots along with the characterizations and modifications of the nanodots photophysical properties. First, the synthetic aspect of this research investigated two combustion-based synthetic pathways of the carbon nanodots. The first pathway (candle-based) utilized the oxidation of candle soot with nitric acid followed by multiple neutralization and separation steps that resulted in carbon nanodots with low quantum yields (~2%). The second pathway (methane-based), utilizing methane gas, produced, and collected the carbon nanodots directly from a flame, which resulted in significantly less experimental time and carbon nanodots with higher quantum yields (~30%). The photophysical characterizations of the synthesized carbon nanodots revealed an excitation wavelength dependent fluorescence, a broad absorption, good photostability, and complex intensity decays. The photophysical properties of the methane-based nanodots were further investigated for a more in-depth understanding of the fluorescence-based structural architecture, utilizing fluorescence quenching methods. These experiments revealed that the fluorescence quenching of the nanodots can occur through both dynamic and static quenching mechanisms depending on the type of quencher utilized. Also, these experiments helped to elucidate the origin of the luminescence of carbon nanodots. While carrying out the quenching experiments, a novel temperature dependent fluorescence property was observed and analyzed, which showed an increase of the emission intensity as a function of increased temperatures. The last aspect of this research was intended to modify the carbon nanodots with bromide for the generation of singlet oxygen (1O2) and to utilize Metal-Enhanced Fluorescence (MEF) to improve the fluorescence and phosphorescence signals from the nanodots. The bromination experiments revealed that the brominated carbon nanodots had some phosphorescence character and could generate low amounts (~1-4%) of singlet oxygen. The MEF experiments showed that both the fluorescence and phosphorescence signal of carbon nanodots could be enhanced when in the presences of silver nanoparticles.